Further studies of the humoral immune response have been undertaken using ovine immunoglobulin G as the immunogen and BALB/C mice as the recipients. Mice immunized by the intraperitoneal route weekly for five weeks with an emulsion of the immunogen in Freund's complete adjuvant and given an intraperitoneal injection of pristane on day 14 produce relatively large volumes of ascitic fluid. A simple fluoroimmunoassay is used to determine specific antibody titres. The mice do not lose condition, remain active and eat normally throughout, although all show extensive granuloma formation within the peritoneal cavity. Tapping is performed before ascitic fluid production is excessive and each study is complete within 49 days, with a maximum of 5 taps. Only a single immunization site is required, since no higher antibody titres are obtained using combined intraperitoneal plus intramuscular plus subcutaneous immunization routes. A dose response study has shown that relatively large amounts (100 micrograms) of ovine immunoglobulin G must be injected to evoke a maximum response. Studies are in progress of various adjuvants and potential modulators of the immune response and of ways in which the immunogenicity of macromolecules can be changed. This approach can also be used to obtain antibodies for diagnostic purposes more rapidly than in rabbits and sheep.